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I I I I I I I I I I I I I I I I I I I Table 2. Comparison of Flood Frequency - Blue River below Dillon Dam Method of Analysis 10-yr 50-yr 100-yr Inflow 2730 3660 4070 (USGS Regression Equations) FIS outflow LP3 outflow (Computed value) 0.05 Confidence limit 0.95 Confidence limit Ratio of inflow to outflow FrS LP3 500-yr 4940 2500 1960 1670 2410 3100 2770 2270 3660 3350 3130 2520 4240 3800 3980 3100 5700 91.6% 71.8% 84.7% 75.6% 82.3% 76.9% 76.9% 80.6% It appears that the FIS hydrology for reservoir outflow assumed a decreasing routing factor for Dillon Reservoir. The available data shows that this is probably not a good assumption since the reservoir volume is relatively fixed so that larger inflows will have less attenuation. The LP3 estimate of outflow follows the pattern of increasing routing factor that is more consistent with reservoir operations. Our understanding of Dillon reservoir operations is that Denver Water draws down the reservoir during the month of May, based on snow pack information. The amount of control they can provide is relatively fixed and depends on the volume of water that can be released in May through the outlet works. As indicated by the LP3 analysis, it is likely that large floods (greater than 100-year) would not be reduced as much as the original FrS analysis indicated. The difference between the LP3 values and the 1978 FIS values is small. The FrS estimate of the 1O-year flood is the only value outside of the confidence limits with a value that is 540 cfs (about 28-percent) higher than the flow computed from the LP3 analysis. The 50- and 1 O-year floods are probably slightly lower than the FrS originally estimated but, because of dam operations, the LP3 analysis is not reliable. The past record of operation indicates that Denver Water cart effectively control a 10-year flood and may be effective up to nearly the 50-year flow. We recommend that the previous FrS hydrology be used for the updated FIS since the two are not significantly different. It is understood, however, that this hydrology appears to be somewhat inconsistent with the operation of Dillon Dam. Current operations by Denver Water probably reduce flood magnitude in the Blue River below what the FIS estimated in 1978 for the 10- and 50-year floods and would not be as effective for controlling the 500-year flood.